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Help with multi LM317 in parallel

dethlore

dethlore

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I may look at strips more if I can't get something figured out for these 5mm.. I have a ton of 5mm which is why I wanted to use em up in a big project, but those strips do look really cool!

So, project aside, I simply put test loads on 2 LM317s in parallel as I have drawn, with 23 Ohms on Vout->Adj, which SHOULD give just shy of 60mA, correct? One LM gave about 58mA and one gave 6mA. I tried 5 LM317s, one at a time, and found that just one gave me 58mA and the other 4 gave 6mA. Is it likely that 4 out of 5 LMs are bad??? They only cost $0.23 each, but I have to wait 2 weeks to get them in unless anyone knows of a place in USA that sells them cheap?
 
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dethlore

dethlore

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Things said:
You are measuring on the Adj pin side and not the Out side correct?
Click to expand...

Yes that is correct. I did try measuring off Vout just in case I was off my rocker, but it read 1.7A! I've built a few similar circuits... I never had this kinda of trouble before. Everything read out normally before. :thinking:
 
T

Things

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Hmm, maybe test your resistors too.

You could have a batch of bad/fake LM317's too, it does happen occasionally.
 
justinjja

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Are you actually putting the IC's in parallel when testing?
This will cause one to do all the work, because the one with the lowest reference voltage will end up pushing all the current,
Test them one at a time

and yes with 23 ohms 5x mA is what you should get

You can get lm317's for expensive at radio shack

or buy them from ebay, just select the option for us sellers only
or buy them direct from mouser/digikey
 
dethlore

dethlore

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I actually did test those... of the two 47 Ohms, one was 47, one was 52.

I thought I had tested these LM317s before and they seemed to work... are they fairly easy to kill? I haven't been doing anything outside of their normal operation (40+volt 1.5+A)

I tested them separately as well as in parallel.
 
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Bionic-Badger

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justinjja said:
Alright here is what you need for 20mA led's

The voltage needs to be atleast 5(or the number of led's in series)*(individual led votlage) + 5v (for lm317 and resistor voltage drop)

by adding a 50 ohm resistor to each string you can help ensure that all led's run at the same current.

also a single properly heatsinked lm317 can run upto 100 strings of 20mA led's

ledsz.jpg
Click to expand...

This circuit involving the LM317 is redundant. You've set the LM317 up in current regulating mode like a "DDL" current regulator; effectively it's regulating current twice in this setup with the current limiting resistors in each strand. If your goal was to produce a reference voltage--for those current limiting resistors below--you need to set the LM317 up as a voltage regulator, not a current regulator.

Things: those cheap LED strings on Amazon and eBay are pretty much what dethlore is making with his parallel circuit. So they'll burn out in the same manner with the parallel circuit.

Dethlore: A bunch of parallel series strands is probably the best and safest solution. The LM317 is a bit overkill for each strand and a simple resistor would do.

Why not also consider cold cathode tubes for lighting?
 
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justinjja

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Bionic-Badger said:
This circuit involving the LM317 is redundant. You've set the LM317 up in current regulating mode like a "DDL" current regulator; effectively it's regulating current twice in this setup with the current limiting resistors in each strand. If your goal was to produce a reference voltage--for those current limiting resistors below--you need to set the LM317 up as a voltage regulator, not a current regulator.

Things: those cheap LED strings on Amazon and eBay are pretty much what dethlore is making with his parallel circuit. So they'll burn out in the same manner with the parallel circuit.

Dethlore: A bunch of parallel series strands is probably the best and safest solution. The LM317 is a bit overkill for each strand and a simple resistor would do.

Why not also consider cold cathode tubes for lighting?
Click to expand...

Honestly it is a simple concept, Without the resistors, you could have uneven current distribution between the strands.

You could just use a properly sized resistor,
but it is not the right way to do it,
Both led forward voltage and the psu output voltage may change under varying circumstances
 
dethlore

dethlore

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Cold Cathode tubes would be pretty neat looking... but you need a power box usually for a pair. Haven't looked inside or know how easy it would be to build my own power inverters for them. I also don't know if you can dim cold cathodes, just use an inline var-resistor?

If I did tubes, I would want one on the left wall, front wall, right wall. 5 colors and 3 levels is 45 tubes... which will be quite a bit more expensive than the 750 LEDs... but I like the "do it once do it right" philosophy.

Alternatively, if I get the right resistors, I can just run a bunch of parallel strings without the LM? 12V would supply full power to 6x red LEDs in series or 3x uv LEDs in series. So just add the proper resistor to each series and away I go?
example:
3 uv LEDs Vo is ~ 12V/20mA, R = 600 Ohm 1/4W ?
6 red LEDs Vo is ~ 12.4V/20mA, R = 600 Ohm 1/4W?

Is this right?

Thanks for your help guys
 
T

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Bionic-Badger said:
Things: those cheap LED strings on Amazon and eBay are pretty much what dethlore is making with his parallel circuit. So they'll burn out in the same manner with the parallel circuit.
Click to expand...

Yes, but each LED on the string has it's own current limiting resistor, so you can get away with simply powering a heap of strings from 12V instead of having to work in series.
 
dethlore

dethlore

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Things said:
Yes, but each LED on the string has it's own current limiting resistor, so you can get away with simply powering a heap of strings from 12V instead of having to work in series.
Click to expand...

I think using the strips would be the best... thinking of not soldering 750 LEDs.... :undecided:

I haven't seen any information on using them, but if I wanted 3 separate strings to separately change color, would I need to get 3 different control boxes/remotes? or is it possible to use one, set the string, and connect it to a different string, set that string, etc.? Should be able to connect multiple strands in parallel right? That's what it's doing every 3rd led anyways isn't it?
 
Bionic-Badger

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justinjja said:
Honestly it is a simple concept, Without the resistors, you could have uneven current distribution between the strands.

You could just use a properly sized resistor,
but it is not the right way to do it,
Both led forward voltage and the psu output voltage may change under varying circumstances
Click to expand...

The voltage won't change if you have the LM317 as the voltage regulator. Generally you'll be running your LEDs at spec current, which means small variations in components won't cause you to exceed its maximum ratings anyway. Your resistors can be purchased cheaply at 1% tolerances too.

Your circuit is actually harmful and can cause the exact sort of catastrophic chain reaction you're seeking to avoid with parallel LEDs. The LM317 does everything in its power to maintain 1.25V between the two outputs. If one of your LED strands get disconnected, and there's voltage headroom above the LM317's input, the regulator is going to pump that current somewhere, which will be the other strands.

That LM317 needs to be placed in a voltage regulation circuit topology--not a secondary current regulation circuit.
 
Bionic-Badger

Bionic-Badger

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Things said:
Yes, but each LED on the string has it's own current limiting resistor, so you can get away with simply powering a heap of strings from 12V instead of having to work in series.
Click to expand...

No, that's the problem. I've inspected some of the strands and they are literally one single resistor at the top, and a bunch of parallel LEDs connected together. There's a reason why those LED strings on Amazon are so cheap!

Maybe some of the better strands are properly wired, but that will require some testing before investing in a lot of strands.
 
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Bionic-Badger

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Cyparagon said:
Give them a break. Resistors are expensive.

oh, wait... ;)
Click to expand...

It's a great opportunity to stock up on parts too. I especially like those quantity deals where the quantity price breaks are actually less than 50% of the previous price like this. Just $9 for 1000! Less than 1 cent per resistor! They used to charge me 5 cents for two at the labs!
 
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